In an engine for driving a working machine such as a small type general engine, typically, the engine revolution speed is appropriately controlled by using a governor mechanism. As the governor mechanism, there is a known mechanism in which a link mechanism is moved by a centrifugal force suitable for the engine revolution speed, the movement of the link mechanism is transmitted to a throttle valve, and the throttle opening is adjusted. However, since an engine used as a power source such as a handheld mower and a blower is used at relatively high speed rotation, stable control result can not be obtained easily by the conventional governor mechanism, and since it is desired to reduce the working machine in size and weight, the engine is not provided with the governor mechanism in many cases.
In an engine having no governor mechanism, the throttle opening of a carburetor is adjusted manually by an operator, thereby controlling the engine revolution speed. However, since the operator adjusts the throttle opening depending upon his or her feeling or guess, the operator tries to enhance the efficiency of the operation and the engine is tend to be used at revolution speed higher than practically usable revolution speed. Further, in recent years, the engine is set to lean combustion mode with emission control, the maximum revolution speed of the engine itself is set higher, and there is a tendency that the engine revolution speed becomes higher together with the afore-mentioned operator's using state. If the engine revolution speed becomes higher, the operation noise caused by the revolution of the engine is abruptly increased, and this is not preferable in terms of vibration and endurance.
In terms of limiting the maximum revolution speed to a predetermined value or lower, Japanese Patent Application Laid-open No. 7-317577 discloses a working machine in which a throttle valve does not come to a fully opened position by mechanically limiting the pivoting range of a throttle operating lever. Further, Japanese Utility Model Application Laid-open No. 54-152813 discloses a mower in which an end of a throttle lever is provided with an adjusting screw so that the engine revolution speed when the throttle lever is fully opened can be increased and reduced.
Patent Document 1: JP 7-317577 A
Patent Document 2: JP 54-152813 U